Ionization gauge and method of using and calibrating same

ABSTRACT

Controller circuitry and method for controlling the operation of an ionization gauge having a source of electrons, an anode, an ion collector electrode. Circuitry is provided (a) for providing an electron emission current from the electron source (b) for measuring the heating power W X  of the electron source to obtain a measured value of the heating power at an unknown pressure P X  and (c) measuring the ion current to the collector electrode to obtain a measured value of the positive ion current i +X  at the unknown pressure. A memory is also provided for storing at least one equation for pressure, the pressure equation being obtained from a reference gauge by measuring the current i +cal  to the positive ion collector electrode, the electron emission current i -cal  from the electron source and the heating power W cal  of the electron source of the reference gauge at selected calibration pressures P cal  and selected heating powers W cal  of the electron source of the reference gauge, the equation for pressure being in the form of P=f(i +  /i - ,W). Moreover, a calculator responsive to the stored equation for pressure and the measured values of positive ion collector current i +X  and heating power W X  of the electron source is provided for calculating a pressure indication P XI  when the predetermined pressure gauge is exposed to the unknown pressure P x  at an emission current i -cal  of the electron source, the pressure indication being calculated according to the equation P XI  =f(i +X  /i -cal , W X ). Means are also provided such that, for a given pressure of the gas and cathode temperature, the temperature of the internal surfaces of the gauge will be substantially repeatable over time and reproducible gauge to gauge.

RELATED PATENTS

This application is related to U.S. Pat. Nos. 5,128,617; 5,250,906;5,296,817; and 5,422,573, all of which are assigned to the assignee ofthe present application and all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to means and a method for accuratelymeasuring pressure with a hot cathode ionization gauge.

2. Discussion of Prior Art

Prior art hot cathode ionization gauges are calibrated by measuring thecurrent i_(+cal) to the ion collector electrode, at a fixed known valueof electron emission current i_(+cal), at known pressures P_(cal), inthe calibration system. A gauge sensitivity S is then defined where##EQU1##

In order to measure an unknown pressure P_(X) in a vacuum system, thecurrent i_(+X) to the ion collector electrode is measured using anemission current value of i_(-cal). It is then assumed that the unknownpressure P_(X) can be calculated using Eq. 2. ##EQU2##

It has long been recognized that Eq. 2 does not give accurate resultsbut surprisingly little has been done to improve the accuracy ofmeasurement.

The root cause of the problem with accuracy is that because ofhistorical precedent, ionization gauges are calibrated in units ofpressure whereas all ionization gauges measure gas density. Underconditions of thermal equilibrium, pressure P and gas density n aresimply related by Eq. 3.

    P=nkT                                                      (3)

where k is the Boltzmann constant, and T is the absolute temperature ofthe gas. However, pressure and density are not simply related variablesin a hot cathode ionization gauge because conditions for thermalequilibrium are not present in an operating hot cathode ionization gaugeand an absolute temperature cannot be defined.

It is instructive to examine in some detail why Eq. 2 does not giveaccurate results.

If Eq. 1 is substituted into Eq. 2, the result is ##EQU3##

The conventional way of interpreting this result is that when

    i.sub.+X =i.sub.+cal                                       (5)

then Eq. 6 must hold.

    P.sub.X =P.sub.cal                                         (6)

This interpretation is the basis for known prior art hot cathodeionization gauge calibrations. However, Eq. 4 also implies that if Eq. 6holds then Eq. 5 must hold. Because the ion collector current in a hotcathode gauge is a function of the gas density n, from Eq. 5 we musthave

    n.sub.X =n.sub.cal                                         (7)

Here, n_(N) is the number of gas molecules per unit volume in the gaugeat the unknown pressure P_(X) and n_(cal) is the number per unit volumepresent in the gauge when the calibration pressure was P_(cal).

If the interior surfaces of the gauge exposed to the ion collectionvolume are not at substantially the same temperature during measurementof P_(X) as during calibration at P_(cal), then the gas moleculesincident on the surfaces will have different average kinetic energy whenthey leave the surfaces, therefore, different average velocity duringmeasurement than was present during calibration. Therefore, the transittime for gas molecules through the ion collection volume will not be thesame during measurement of P_(X) as during calibration. If the transittimes are not the same, then the number of molecules per unit volumewhich are present will not be the same and Eq. 7 is not satisfied.

For Eq. 7 to be satisfied requires that the average energy of the gasmolecules in the ion collection volume during measurement of an unknownpressure P_(X) be substantially the same as that prevailing duringcalibration at substantially the same pressure. If the average energy isdifferent, then Eq. 7 is not satisfied.

There is considerable prior art on how to correct pressure measuringtransducers for the effects of temperature changes. See, for example,U.S. Pat. No. 4,468,968 wherein it is taught how to correct for theeffects of temperature change on the transducer elements per se but noton changes in the medium being measured. In a hot cathode ionizationgauge ambient temperature changes have negligible effect on theperformance of the gauge itself but can have substantial effects on thegaseous medium being measured and, therefore, on the output of theionization gauge.

In U.S. Pat. No. 4,866,640, Morrison teaches that the effect of adifferent gas temperature during use than was present during calibrationof a hot cathode ionization gauge can be corrected for from gastemperature measurements. The ratio of the absolute gas temperature,T_(cal), measured during calibration divided by the absolute gastemperature, T_(use), measured during use is multiplied by the value ofgauge sensitivity, S_(cal), obtained during calibration to obtain acorrected value of gauge sensitivity, S_(use). See Eq. 16 in Morrison.

A fundamental error in the teachings of Morrison in U.S. Pat. No.4,866,640 is the assumption that an absolute gas temperature can bedefined in a hot cathode ionization gauge. The gas temperature as usedby Morrison can only be defined for conditions of thermal equilibrium.However, thermal equilibrium is not present in a hot cathode gauge wherethere is net heat flow between numerous parts.

Another error is the assumption that a gas temperature can be measuredpractically in a hot cathode gauge. Although Morrison specifies a gastemperature measuring element, there is no teaching of how the gastemperature can be measured at the low pressures of interest where themass of all the gas in the gauge is orders of magnitude below that ofany known temperature sensor. For example, at 1×10⁻¹⁰ Torr the totalmass of gas in a gauge is only of the order of 10⁻¹⁴ gram. An equal massof tungsten would have a volume of approximately 10⁻¹⁵ cm³. The heatcontent of all of the gas in the gauge at low pressure is orders ofmagnitude less than that in the smallest gauge part and will have noeffect on a practical gas temperature sensor.

Morrison also ignores the presence of radiated energy from theincandescent cathode. All surfaces within a hot cathode gauge are bathedin radiant energy from the hot cathode which will affect the temperatureof any gas temperature sensor many orders of magnitude greater than willthe relatively few gas molecules present at low pressure. For thisfurther reason, gas temperature cannot be measured in a hot cathodeionization gauge in a practical way.

Another error is that Eq. 3 applies in a hot cathode ionization gauge.Equation 3 above holds only under conditions of thermal equilibrium andthus Morrison is in error in using this simple relationship betweenpressure and temperature in a hot cathode ionization gauge where thermalequilibrium does not exist.

In U.S. Pat. No. 5,250,906 where one of the inventors is the presentapplicant, claim 8 thereof recites, inter alia, that a reference gaugeused during calibration has the same sensitivity at any given pressureand cathode heating power as a predetermined gauge used to measureunknown pressure.

However, applicant has found numerous instances in which the referencegauge had substantially the same sensitivity at any given pressure andcathode heating power as did the predetermined gauge at one time and notat other times. Thus, U.S. Pat. No. 5,250,906 does not teach how tocause the predetermined gauge to behave consistently like the referencegauge or vice versa.

The concept of gauge sensitivity S has been universally used in priorart gauge calibration methods with and without corrections to S forchanges in surface temperatures within the gauge. Applicant hasdiscovered that S is a complicated function of the temperature and areaof the surfaces exposed to the gas in an ionization gauge and,therefore, is not an appropriate parameter for use in accurate pressuremeasurement.

Furthermore, applicant has found a new method of calibrating hot cathodeionization gauges which completely avoids the use of the concept ofgauge sensitivity and any need to measure gas temperature, therefore,avoids the complications introduced into pressure measurement when Schanges.

SUMMARY OF THE INVENTION

The present invention consists of apparatus and a method for accuratelymeasuring pressure with an ionization gauge.

The apparatus consists of an ionization gauge such as described in U.S.Pat. No. 5,128,617 wherein the path lengths of the electrons emittedfrom the cathode have purposely been made substantially repeatable overtime in the same gauge and reproducible gauge to gauge and wherein allheat flow paths and surface emissivities have purposely been modesubstantially repeatable within a given gauge and reproducible gauge togauge, plus means for conventionally operating the gauge, plus means formeasuring cathode heating power, plus means for storing one or moreequations, plus means for calculating an unknown pressure frommeasurements of the ion collector current, emission current, and cathodeheating power.

The equation(s) is of the form

    P=f(i.sub.+ /i.sub.-, W)                                   (8)

where i₊ is the positive ion current to the ion collector electrode

i₋ is the emission current

W is the cathode heating power

Equation 8 is obtained by measuring i_(+cal), i_(-cal), and W_(cal) at aseries of known calibration pressures P_(cal) and heating powers. Theheating power required to give a constant emission current i_(cal) canbe caused to vary by introducing small amounts of oxygen or otherwisecontaminating the cathode which serves to raise the work function of thecathode. This calibration data is plotted in three dimensions usingreadily available three dimensional plotting software to obtain a bestfit equation for the calibration data. Equation 8 is stored inionization gauge controller memory. An unknown pressure can then bemeasured with the calibrated gauge simply by measuring i_(+X), i_(cal),and W_(X), substituting in Eq. 8 and calculating a pressure indicationP_(XI) corresponding to the unknown pressure P_(X).

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a block diagram of illustrative apparatus for measuring gaspressure with an ionization gauge wherein an improved calibrationtechnique in accordance with the invention may be utilized.

FIGS. 2a-2f are schematic diagrams of illustrative joints between priorart gauge parts.

FIGS. 3a-3c are schematic diagrams of preferred joints between gaugeparts in accordance with the invention.

FIG. 3d is a schematic diagram which illustrates preferred electronemissive surfaces gauge to gauge in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Reference should be made to the figures where like reference numeralsrefer to like parts.

In FIG. 1 is shown apparatus 10 for measuring gas pressure accuratelywith an ionization gauge. The ionization gauge 14 is attached to avacuum system 12 whose pressure is to be measured. The ionization gauge14 is of a stable design such as described in U.S. Pat. Nos. 5,128,617;5,250,906; 5,296,817; or 5,422,573 with improvements as described below.Gauge 14 is operated in a conventional manner by a power supply 18 whichsupplies well-controlled bias voltages to the anode and cathode of thegauge together with well-controlled power to heat the cathode to thetemperature required to obtain the desired electron emission. Thecathode heating power is measured by the power measurement circuitry 20.The cathode heating power may also be calculated from the cathodevoltage and current as described in U.S. Pat. No. 5,250,906. The currentto the ion collector electrode of gauge 14 is measured by ion currentmeasurement circuitry 22. The electron emission current from the cathodein gauge 14 may be measured by electron emission current measurementcircuitry 24 or may be a parameter entered by the user as described inU.S. Pat. No. 5,250,906.

The cathode power signal W from measurement circuitry 20 is fed intocalculation circuitry 30 together with the i₊ signal from measurementcircuitry 22 plus the i₋ signal from measurement circuitry 24.

Equation 8 is stored in the memory 28 for use in calculating pressureindications P_(I) which are output by the calculation circuitry 30.Equation 8 may be obtained for a given ionization gauge 14 as follows. Areference gauge is attached to a conventional vacuum calibration systemand calibrated at a series of known calibration pressures P_(cal) undervery clean conditions as is well-known in the art. With very cleanconditions existing in the gauge, cathode heating power is minimal. Thecurrent, i_(+cal), to the ion collector is measured and recorded atconstant emission current, i_(+cal) for each selected pressure. Inaddition, the cathode heating power W_(cal) is measured and recorded foreach selected pressure. Then this calibration process is repeatedmultiple times with higher cathode heating powers. Higher cathode powercan be achieved by briefly exposing the hot cathode to a minute amountof a contaminating gas such as oxygen. As described in U.S. Pat. No.5,250,906, it is well known that traces of oxygen raise the workfunction of the emitting surface. After oxygen contamination, a highercathode temperature is required to achieve the desired emission current.A higher cathode temperature requires increased cathode heating power.

The data sets for each cathode power selection are then used to generatea three dimensional surface which best fits the data using commerciallyavailable software. In some cases, a better fit can be obtained if thepressure range is divided up so that multiple equations result. Theequation(s) 8 is stored in memory 28 for later use.

To use a gauge 14 to measure an unknown gas pressure where gauge 14 maycorrespond to either (a) a gauge calibrated as described above or (b) anon-calibrated gauge, the gauge 14 is attached to the vacuum system 12where the gas pressure is to be measured as is well-known in the art. Atan unknown pressure P_(X) in the system, the current i_(+X) to the ioncollector electrode is measured together with the fixed emission currenti_(-cal) and the cathode heating power W_(X). The signals i_(+X),i_(-cal) and W_(X) are sent to the calculation circuitry 30. Equation 8is also sent to calculation circuitry 30 from memory block 28. In thecalculation circuitry 30, these signals are used in Eq. 8 to calculatean indicated pressure signal P_(IX) which closely corresponds to theunknown pressure P_(X).

The foregoing calibration technique can be utilized in accordance withthe present invention with non-calibrated gauges because the interiorsurfaces of the gauge exposed to the ion collection volume are atsubstantially the same temperature during measurement of the unknownpressure as are the interior surfaces of the reference gauge duringcalibration at the calibration pressures. Thus, the average energy ofthe gas to molecules in the ion collection volume during measurement ofthe unknown pressure is substantially the same as that prevailing duringcalibration at substantially the same calibration pressure. This, aswill be described in further detail below with respect to FIGS. 3athrough 3d, can be implemented by making all heat flow paths and surfaceemissivities substantially repeatable within a given gauge andreproducible from gauge to gauge. Thus, the heat flow paths and surfaceemissivities in the reference gauge used during calibration and thepredetermined gauge used to effect measurement of the unknown pressureare substantially the same. Moreover, the path lengths of the electronsemitted from the cathode are preferably made substantially repeatableover time in the same gauge and the reproducible gauge to gauge, asdescribe in U.S. Pat. No. 5,128,617.

In general, applicant has discovered that in an ionization gauge, if theheat flow paths from electrodes to supports, from supports to vacuumenclosure, from shield to vacuum enclosure, from enclosure to system,etc., (where the latter elements are known and illustrated in U.S. Pat.Nos. 5,128,617 and 5,250,906, for example) are made repeatable over timein the same gauge and reproducible gauge to gauge, considerable benefitscan be gained. Although in U.S. Pat. No. 5,128,617, the need to maintainstable geometry and electron paths is taught to achieve long termaccuracy and reproducible behavior gauge to gauge, the need to maintainstable heat flow within a given gauge and reproducibility gauge to gaugehas not been previously recognized. This fact is understandable becausethe additional accuracy provided by stable heat flow paths would havebeen unrecognizable in the presence of the large error in pressureindication in ion gauges before the teachings of U.S. Pat. No. 5,128,617were implemented.

In FIGS. 2a through 2f are shown illustrations of typical prior artjoints including spot welds 44 indicated by an "X". In FIG. 2a there areillustrated joints between gauge elements 40 and 42 where the contactarea is variable and changes with time. In particular, due to repeatedheating and cooling of the joint between elements 40 and 42, the element42 tends to peel from the element 40 as illustrated in the lower portionof FIG. 2a. Thus, the contact area between elements 40 and 42 varieswith time whereby a stable heat flow within a given gauge andreproducibility from gauge to gauge can not be maintained.

With respect to FIGS. 2a through 2d, it should be noted that element 42may, for example, correspond to the cathode filament or to an electrodesuch as the anode or ion collector while element 40 may correspond toone of the supports for these elements. Alternatively, element 42 maycorrespond to one of the supports for the foregoing elements whileelement 40 corresponds to a pin or the like disposed in the vacuumenclosure of the gauge where in the latter instance, the elements 40 and42 would be more approximately equal in size.

Further, it should be noted that the heat flow direction illustrated inFIG. 2a corresponds to the heat flow direction found in ionizationgauges where heat is conducted from the gauge elements 40 or 42 to thesystem. As can be appreciated, with peeling of gauge element 42 withrespect to gauge element 40, the contact area between these elementsvaries thus effecting the heat flow between these elements and thuschanging the temperature distribution on the interior surfaces of thegauge.

Referring to FIG. 3a, there is illustrated a technique in accordancewith the invention to avoid the problem introduced when element 42 peelswith respect to element 40 or vice versa. In particular, as shown in theupper portion of FIG. 3a, element 42 (or element 40) may be providedwith a protrusion or bump 46 whereby the contact area between elements40 and 42 is minimized such that spacings 48 and 50 occur betweenelements 40 and 42. With the minimization of the contact area providedby bump 46, the tendency of the elements to peel with respect to oneanother is substantially lessened. As can be seen from FIGS. 3a and 3b,spot weld 44 is provided at the bump to effect the joint betweenelements 40 and 42 where the lower portion of FIG. 3a shows the elements40 and 42 and spot weld 44 in plan view.

Referring to FIG. 2b, there is illustrated another problem which occursin prior art gauges wherein the heat flow path length is not maintainedconstant, the heat flow path length being the entire length of the heatpath including, for example, (a) the cathode filament, its supports, andthe vacuum enclosure pins to which its supports are connected; (b) theanode, its supports, and the enclosure pins to which its supports areconnected; or (c) the ion collector electrode, its supports, and theenclosure pins to which its supports are connected where again thelatter elements are well known in the art.

As can be seen in FIG. 2b, the spot weld 44 for a gauge A illustrated inthe upper portion of FIG. 2b is at a different location than the spotweld 44' for the gauge B illustrated in the lower part of FIG. 2b wherethe gauge A, for example, may correspond to the reference gauge usedduring calibration and the gauge B may correspond to a non-calibratedgauge used to measure an unknown system gas pressure or gauges A and Bmay both be non-calibrated gauges. These different positions of the spotwelds 44 and 44' are indicated as a difference in the heat flow pathlengths of the gauges A and B. Again, this difference in heat flow pathlength will affect the temperature distribution on the surfaces of thegauge elements 40 and 42 such that the temperature distribution on thesurfaces of these elements will be different for gauge A compared to thetemperature distribution on the surfaces of these elements of gauge B.

The foregoing problem can be overcome, as illustrated in FIG. 3b, byinsuring that the spot welds 44 and 44' which connect the elements 40and 42 of the upper and lower gauges of FIG. 3b are located the samedistance a from the ends 48 and 50 of the gauge element 40 of the gaugesA and B. Moreover, the bumps 46 will also be located the same distancefrom the ends 48 and 50 of the gauge elements 40. Hence, in theforegoing manner, stability of heat flow paths is further provided.

Referring to FIG. 2c, there is illustrated a further problem with priorart gauges in that the size of the spot weld utilized in gauge A isdifferent from that utilized in gauge B and this difference in contactarea of the spot welds 44 and 44' will further contribute to instabilityof the heat flow paths in the gauge.

Moreover, referring to FIG. 2d, the number of spot welds 44 utilized ingauge A differs from the number utilized in gauge B and thus again theheat flow area has not been maintained constant due to a difference inthe contact areas of the spot welds. These differences in the contactareas of the spot welds, as illustrated in FIGS. 2c and 2d, again resultin the inability to maintain stable heat flow within a given gauge andreproducability gauge to gauge. This problem is overcome in the presentinvention, as illustrated in FIGS. 3a and 3b, where a single spot weld44 may be employed and where the size of the spot weld is the same forboth gauges A and B.

Referring to FIGS. 2e and 2f, there are illustrated gauge elements 51and 52 of gauges A and B where the element 51 may correspond to theshield illustrated as element 12 in U.S. Pat. No. 5,250,906 and theelement 52 may correspond to the vacuum enclosure 24 illustrated in theforegoing patent. The element 51 is typically connected to element 52where differences in contact area between elements 51 and 52 in gauges Aand B result due to the differing amounts of penetration of weld beads54 and 56, as can be seen in FIGS. 2e and 2f. This problem can beovercorme as illustrated in FIG. 3c where there is a completepenetration of the weld bead 58 in all gauges and thus the contact areabetween elements 51 and 52 is maintained constant within a given gaugeand is reproducible from gauge to gauge.

Another problem with respect to prior art gauges is that the emissivityof the hot cathode is not reproducible from gauge to gauge because thearea covered by the cathode coating is not reproducible from gauge togauge. Referring to FIG. 3d, there are illustrated cathode coatingscovering substantially the same areas of the cathodes. In particularcathode coatings 60 of gauges A and B cover substantially equal surfaceareas of the filament wires 62 and thus the emissivities of the hotcathodes of gauges A and B are substantially the same. Again, this alsoresults in stable heat flow paths which are reproducible from gauge togauge.

By implementing one or more of the features of FIGS. 3a through 3d,steps can be taken to effectively assure that, for a given pressure ofthe gas and cathode temperature, the temperature of the internalsurfaces of the gauge will be substantially the same over time and fromgauge to gauge. If the internal surface temperatures are repeatable overtime and reproducible from gauge to gauge at any given pressure andcathode temperature, then the average energy of the molecules, the gasdensity and, therefore, i₊ /i₋ will be repeatable and reproducible. Ifi₊ /i₋ is repeatable and reproducible then the pressure indication P_(I)will be repeatable and reproducible.

What is claimed is:
 1. An ionization gauge comprising gauge elementsincluding:a source of electrons; an open anode defining an anode volume;and a collector electrode for collecting ions formed by impact betweensaid electrons and gas molecules within said anode volume so that thepressure of the gas can be measured to provide a measurement output ofthe cauge; where at least one of the gauge elements includes aprotuberance where the gauge element is spot welded to another gaugeelement such that contact between the gauge elements occurssubstantially only at said protuberance.
 2. An ionization gauge as inclaim 1, where a first member of the pair of spot-welded gauge elementsis the anode, electron source, or collector electrode and a secondmember of the pair of spot-welded gauge elements is a respective supportfor the first member.
 3. A plurality of ionization gauges, eachcomprising gauge elements including:a source of electrons, an open anodedefining an anode volume; and a collector electrode for collecting ionsformed by impact between said electrons and gas molecules within saidanode volume so that the pressure of the gas can be measured to providea measurement output of the gauge; where at least one of the gaugeelements includes a protuberance where the gauge element is spot weldedto another gauge element such that contact between the gauge elementsoccurs substantially only at said protuberance.
 4. A plurality ofionization gauges as in claim 3, where the distance of the spot weldfrom the end of one of said gauge elements of one of said gauges issubstantially the same distance as the location of the spot weld fromthe end of the corresponding one of said gauge elements of another oneof said gauges.
 5. A plurality of ionization gauges having respectivespot welds as in claim 3, where the sizes of said respective spot weldsare substantially the same.